Spark gap trigger circuit



July 14, 1964 GQDLQVE 3,141,111

SPARK GAP TRIGGER CIRCUIT Filed June 22, 1961 TlME - i }--s NANO sec GAPOUTPUT GAP VOLTAGE INVENTOR TERRY F. GODLOVE ATTORNEY United StatesPatent O 3,141,111 SPARK GAP TREGGER CIRCUIT Terry F. Godlove, Suitland,Md, assignor to the United States of America as represented by theSecretary of the Navy Filed June 22, 1961, Ser. No. 119,261

3 Claims. (Cl. 315-181) (Granted under Title 35, U.S. Code (1952), sec.266) The invention described herein may be manufactured and used by orfor the Government of the United States of America for governmentalpurposes without the payment of any royalties thereon or therefor.

The present invention is directed to an improved auxiliary spark triggergap and more particularly to a simple trigger gap circuit capable ofcausing breakdown in a main switch gap at voltages below normalbreakdown threshold.

Heretofore various methods and systems have been employed -to facilitatebreakdown of a main gap switch in an electrical circuit; however, thesesystems have their drawbacks with deleterious effects on the switchesdue to the trigger sparks. Prior art systems use main switch spark gapswhich are triggered by a high-voltage pulse applied to one of theelectrodes or to a trigger pin in one electrode. Such systems usetrigger pins imbedded in the cathode electrode or a trigger pin securedin the spacing between the electrodes of the main circuit with a highvoltage pulse applied thereto. Another system makes use of a threeelectrode switch which is triggered by means of carefully adjusting theovervoltage on each gap. In the use of trigger pins, there is a stronginteraction between the trigger circuit and the main gap circuit. Inswitches where the trigger pin is located in the center of the main gap,the spark is visibly distorted by the presence of the trigger pin andcontacts the trigger pin. Where the trigger electrode is located in ahole in one electrode, the spark will travel from one electrode of themain gap to the trigger electrode and then to the other electrode of themain switch. This energy transfer takes the form of transients oroscillations superimposed on the main gap output pulse which formdepends on the detailed circuit conditions.

The present invention overcomes the drawbacks of the prior art bytriggering a spark gap without the use of triggering pins secured on themain switch electrodes. This invention makes use of a source of intenseultraviolet light having a copious emission in the wavelength regionwhere the maximum number of cathode photoelectrons can be produced, i.e.a wavelength of about 1100 Angstrom units. The duration of the light totrigger a main gap switch need be between about 6 nanoseconds to aboutnanoseconds depending on the rise time limitation which depends on thesize of resistors and capacitors in the spark trigger gap supply line.The present invention has-a pulse rise time which permits gap breakdownin a time corresponding to one avalanche.

It is therefore an object of the present invention to provide a sparkgap trigger which is relatively simple with good time stability.

Another object is to provide a spark gap trigger which yields a usefulstable voltage range in the main gap circuit.

Still another object is to provide a spark gap trigger which can be usedin open construction as well as in a gaseous medium.

Yet another object is to provide a spark gap trigger which has noelectrical interaction between the spark gap trigger circuit and themain switch gap circuit.

Other and more specific objects of this invention will become apparentupon a more careful consideration of 3,141,111 Patented July 14, 1964the following detailed description when viewed together with theaccompanying drawing, in which FIG. 1 illustrates a spark gap triggercircuit positioned relative to a main gap switch of the main circuit;and

FIG. 2 illustrates in the lower curve the gap voltage vs. time appliedto the trigger gap in the upper curve the voltage (or current) deliveredto the load of the trigger gap circuit.

The present invention makes use of a simple trigger gap circuit which iscapable of producing a very high rate of ultraviolet photon emission ata power of about 0.02 Joule. The trigger gap is positioned relative tothe main gap switch such that photons produced in the trigger gap strikethe cathode of the main gap switch and flood the gap between theelectrodes. The photons incident on the main gap switch causes voltagebreakdown across the main switch gap spacing to control the maincircuit. It has been determined that a breakdown can be caused in a timeof the order of one electron crossing time, thereby giving a timestability in the nanosecond (10 sec.) region and with operation of themain circuit at voltages below the normal breakdown threshold.

Now referring to the drawing there is shown for illustrative purposes inFIG. 1 a simple electrical spark gap triggering circuit with the sparkgap of the triggering circuit positioned relative to the switch gap ofthe main electrical circuit. The spark gap triggering circuit includes a6587 or 5022 thyratron waveform generator 11 which is controlled by grid12 that has a grid leak resistor 13 connected between the grid andground and a charging resistor 14 connected between the load and theanode 15 with the cathode 16 connected to ground. The anode is connectedto the center wire 17 of a coaxial cable transmission line 18 through acapacitor 21 and a resistor 22 connected in series, respectively. Aresistor 23 is connected to provide a ground return for one side ofcapacitor 21. This resistor 23 is connected to the conductor line fromthe cathode to the outer conductor 24 of the coaxial cable 18. At theopposite end of the coaxial cable 18, one electrode 25 of a spark gapconnects with the center conductor 17, and the outer conductor 24connects with the outer conductor 26 of another transmission line 27wherein the center conductor 28 of the transmission line 27 connectswith the other electrode 29 of the spark gap triggering switch. Anattenuator 30 is connected at the other end of transmission line 27,whose impedance matches that of the transmission lines 18 and 27.

The main circuit as shown has an applied source, V, which connects to acoaxial cable 31 through a charging resistor 32. The opposite end of thetransmission line connects with the main spark electrodes 33 and 34through a load resistor 35 wherein the load resistor has an impedancewhich matches that of the transmission line 31. The main spark gapelectrodes are made of brass, stainless steel, or any other suitablematerial. The main gap circuit is typical and is for illustrativepurposes only. Other main circuits such as a capacitor bank circuit mayequally well be triggered with the triggering circuit of the presentinvention. The triggering spark gap switch is positioned relative to themain gap switch such that the distance is less than two inches and at anangle of about 70 degrees or less with respect to the axis of the maingap switch electrodes such that ultraviolet light produced by thetrigger switch shines onto the negative electrode of the main switch.

FIG. 2 illustrates gap voltage vs. time for the voltage breakdown acrossthe triggering gap circuit. The lower line illustrates the curve inwhich no breakdown occurs whereas the upper curve illustrates the pulseoutput or the voltage delivered to the attenuator 30.

For operation, the basic idea is to overvoltage the triggering gap in atime shorter than the time it takes to break down under the applied peakvoltage. The limiting breakdown time of the type 6587 hydrogen thyratronhas been determined to be about nanoseconds with a shield to reduce leadinductance. Thus the rise of voltage on the trigger gap depends on theresistor 22, and the combination of capacitor 21 and the cable (18)capacitance all in series. A capacitance of 67 micromicrofarads and aresistance of 100 ohms is adequate for most applications. Higherresistance can be used for applications requiring the ultraviolet lightto be cut off sharply whereas lower resistance may result in deleteriousmultiple reflections in line 18. The stored energy of the capacitor isjust greater than the energy required to produce the pulse across thetrigger gap electrode. The time required for breakdown of the triggergap depends on the overvoltage with a preferred overvoltage of at least50100%. This can be assured by reducing the spacing of the trigger gap,if necessary. It is necessary that peak voltage be reached prior tobreakdown, however, the required jitter and stability dictates theamount of overvoltage and the circuit parameters. In any case the risetime of the ultraviolet producing pulse must be less than the timecorresponding to one electron drifting across the main gap spacing forgood timing stability.

The main supply voltage V should be adjusted to be below the normalsparking threshold of the main gap, called V which threshold depends onthe main gap spacing used. Denoting the difference V V byAV then thefractional reduction AV/V may be set from about 1% to about 7% for airand up to about if the gaps are enclosed and oxygen is used. The greaterthe percentage reduction the greater will be the time delay between thetrigger spark and the main spark, hence the greater the timinguncertainty. A reduction of 2% to 5% will be most widely applicable.

The trigger gap of the present invention can be used for systems whichrequire an accurate timed firing of a main source line which may use lowor high repetition rates. For the purposes of this invention the systemwill be described in combination with a pulse generator which suppliespulses of current at a fixed repetition rate to develop a pulsedmagnetic field which bends a beam of electrons as they enter a donut.

In operation, a separate voltage supply is applied to the trigger gapcontrol circuit and the main circuit. The voltage applied to the controlgrid is changed to trigger the thyratron tube. Triggering the thyratrontube produces a high voltage pulse on the trigger gap. This high voltagepulse on the trigger gap causes it to break down and produce a source ofultraviolet light, some of which is incident on the cathode of the maingap switch. The ultraviolet light must be of sufiicient intensity toproduce a large number of photoelectrons in the main switch gap. Thesephotoelectrons by means of space-charge distortion of the field cause avoltage breakdown between the main electrodes which permits the maincircuit to operate. Space charge distortion is necessary to causebreakdown at voltages below normal threshold. An electrical pulse willdevelop across the load of the main circuit and when the discharge iscomplete the voltage across the main gap is not sufiicient to maintainthe current in the main circuit. After the main gap recovers anotherpulse from the trigger gap circuit triggers the operation of the maincircuit to cause another pulse across the load. This operation iscontinued with a repetition rate of about 10 per second which depends onthe capacitance buildup time in the main circuit. The trigger circuit iscapable of operating up to about 500 pulses per second with the circuitshown and higher with a smaller resistor 14. The pulses in each circuitare shaped by using the principle of reflection from an open-endedtransmission line and the length of the transmission line of the maincircuit must be of a length which will maintain a pulse length somewhatlonger than the rise time of the main output pulse. The intense pulseproduced across the trigger gap has a short rise time which producesultraviolet light that produces photoelectrons in the main gap whichcauses breakdown of the main gap by distortion of the electric fieldeven when the main supply voltage is set below normal threshold on themain switch gap.

The operation has been described for use in the atmosphere where theelectrodes of the trigger gap are positioned adjacent to the main gap.The switches can be operated in air, oxygen, hydrogen or nitrogen withthe best results obtained with operation in oxygen. The trigger gap canbe positioned such that the ultraviolet light shines through the anodeonto the cathode or any other arrangement. The important factor is thatthe ultraviolet light shine on the cathode with the highest possibleintensity. The gaps can usually be shaped to permit close proximitybetween the trigger spark and the cathode.

The addition of a small point 40 on the positive electrode of thetrigger gap reduces somewhat the uncertainty in the time of firing ofthe trigger gap.

The present invention can be used with various types of equipment whichrequire repetitive or non-repetitive triggering with improved results inpulse shaping and in pulse rise time. The rise time and pulse shape ofthe main circuit pulse depend only on the main circuit and the gas usedand are not aifected by the operation of the trigger circuit, because ofthe electrical isolation provided by this invention.

A subsidiary advantage is that if there are any auxiliary circuits whichrequire an electrical triggering pulse to be actuated, such a triggercan be obtained directly from the output of the attenuator 30 whichterminates the transmission line 27 of FIG. 1. This trigger pulse is adirect result of the breakdown of the trigger gap, hence has more timingcertainty than the pulse applied to the grid of the hydrogen thyratron.

Obviously many modifications and variations of the present invention arepossible in the light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims the inventionmay be practiced otherwise than as specifically described.

What is claimed is:

1. A spark gap triggering circuit comprising a voltage source, anelectronic switch connected thereto, a spark gap switch comprising apair of spaced electrodes, an electrical circuitry comprising a seriescapacitor and resistor and a coaxial cable transmission lineelectrically connected between said electronic switch and said spacedelectrodes of said spark gap switch, said electrical circuitry producinga high voltage pulse across said spaced electrodes, said high voltagepulse producing ultraviolet light of high intensity and for a period oftime of the order of magnitude of the time corresponding to one electrondrifting across a main gap switch of a main circuit whereby said maingap switch is triggered by said ultraviolet light.

2. A spark gap triggering circuit as claimed in claim 1 wherein saidspark gap switch electrodes are separated by a distance of from about A;inch to about 2 inches.

3. A spark gap triggering circuit as claimed in claim 2 wherein saidspark gap switch is positioned relative to the main circuit switch suchthat the angle between the ultraviolet light and the axis of the mainswitch electrodes is about 70.

References Qited in the file of this patent UNITED STATES PATENTS2,405,071 Tonks July 30, 1946 2,931,948 Forgacs Apr. 5, 1960 2,979,640Edmonson et al. Apr. 11, 1961 3,014,156 Osterhammel et a1. Dec. 19, 1961

1. A SPARK GAP TRIGGERING CIRCUIT COMPRISING A VOLTAGE SOURCE, ANELECTRONIC SWITCH CONNECTED THERETO, A SPARK GAP SWITCH COMPRISING APAIR OF SPACED ELECTRODES, AN ELECTRICAL CIRCUITRY COMPRISING A SERIESCAPACITOR AND RESISTOR AND A COAXIAL CABLE TRANSMISSION LINEELECTRICALLY CONNECTED BETWEEN SAID ELECTRONIC SWITCH AND SAID SPACEDELECTRODES OF SAID SPARK GAP SWITCH, SAID ELECTRICAL CIRCUITRY PRODUCINGA HIGH VOLTAGE PULSE ACROSS SAID SPACED ELECTRODES, SAID HIGH VOLTAGEPULSE PRODUCING ULTRAVIOLET LIGHT OF HIGH INTENSITY AND FOR A PERIOD OFTIME OF THE ORDER OF MAGNITUDE OF THE TIME CORRESPONDING TO ONE ELECTRONDRIFTING ACROSS A MAIN GAP SWITCH OF A MAIN CIRCUIT WHEREBY SAID MAINGAP SWITCH IS TRIGGERED BY SAID ULTRAVIOLET LIGHT.